1. Field of the Invention
The present disclosure generally relates to the field of integrated circuit manufacture, and, more particularly, to a method and a system for immersion lithography.
2. Description of the Related Art
The formation of various integrated circuit (IC) structures on a wafer often relies on lithographic processes, sometimes referred to as photolithography or simply lithography. As is well known, lithographic processes may be used to transfer a pattern of a photomask (also referred to herein as a mask or a reticle) to a wafer.
For instance, a pattern may be formed from a photoresist layer disposed on the wafer by passing light energy through a reticle having an arrangement to image the desired pattern of the integrated circuit onto the photoresist layer. As a result, the pattern is transferred to the photoresist layer. Usually, after exposing the photoresist layer, a development cycle is performed, e.g., by an appropriate heat processing of the wafer. In areas where the photoresist is sufficiently exposed, the photoresist material may become soluble such that it may be removed to selectively expose an underlying layer, e.g., a semiconductor layer, a metal or metal-containing layer, a dielectric layer, a hard mask layer, etc. Portions of the photoresist layer not exposed to a threshold amount of light energy will not be removed and serve to protect the underlying layer during further processing of the wafer. Further processing of the layer may include, but is not limited to, etching exposed portions of the underlying layer, implanting ions into the wafer, etc. Thereafter, the remaining (protecting) portions of the photoresist layer may be removed.
There is an ongoing trend in IC fabrication to increase the density of the structures of the IC and, in particular, the density of the individual elements. As a result, there is a corresponding need to increase the resolution capability of lithography systems. Generally, the resolution of a lithography system is limited at least in part by the wavelength used to expose portions of the photoresist. In particular, the finest resolvable or critical dimension is proportional to the wavelength of the light used for exposure. Another limiting factor is the numerical aperture, the critical dimension being inversely proportional to the numerical aperture. Accordingly, there has been a trend to reduce the wavelength as well as increase the numerical aperture.
In modern systems, the reticle is not directly projected onto the wafer, but rather an imaging system is used to project the exposure pattern onto the wafer. Such an imaging system is capable of reducing the size of the structure defined by the reticle by a certain factor, e.g., a factor of 4 to 6. However, the imaging system cannot overcome the limitations imposed by the wavelength and the numerical aperture. Therefore, in order to further increase the resolution of the lithographic system, as an alternative to conventional “dry” lithography systems, an immersion lithography system has been proposed wherein the light is not transmitted to air or a vacuum from the imaging system to the wafer, but rather to an immersion lithography medium. Such an immersion lithography medium may be purified in de-ionized water for use in conjunction with a 193 nm light source, e.g., an argon fluorine (ArF) laser. For other wavelengths, other immersion mediums may be suitable. These immersion mediums replace an air or gas gap that is conventionally present between the final optical component of a conventional dry lithography imaging system and the wafer. However, attempts to implement immersion lithography have encountered a number of challenges. For example, minor variations or non-uniformities in the index of refraction of the immersion medium can adversely affect the quality of the exposure pattern incident on the wafer. The causes of change in the index of refraction of the immersion medium may include, for example, flow of the immersion medium, changes in the density of the immersion medium, changes in temperature of the immersion medium and so forth. A variation in temperature of the immersion medium may arise from part of the exposure radiation that is absorbed by an immersion medium. Further, since the immersion medium is in contact at least with the wafer, or with the wafer and the imaging system, heat may be transferred from the wafer to the immersion medium and/or from the imaging system to the immersion medium.
Usually, a wafer contains a plurality of individual devices. In order to increase the resolution and the quality of the exposure of the photoresist, a reticle is usually not configured for exposure of the whole wafer, but rather for exposure for only a part of the wafer. For example, the reticle may include the structure of one device or a few devices, e.g., four devices (dies). This means that, for exposing the whole wafer, the imaging system and the wafer have to be moved with respect to each other in order to expose the whole wafer step by step. Accordingly, systems of this type are usually referred to as steppers. Some steppers expose the whole reticle in one shot, whereas other types of lithographic systems only expose the whole width of the reticle at one time and exposure of the whole reticle is done by scanning the reticle in a direction perpendicular to the width.
It has been proposed to immerse the substrate in the lithographic system in the immersion medium having a relatively high refractive index, e.g., water. Other immersion liquids have been proposed, including water with solid particles, e.g., quartz, suspended therein. Still other systems propose the use of a conforming immersion medium such as a medium having a solid, semi-solid, gel-like or rubbery consistency. Submersing the wafer in a bath of liquid means that there is a large body of liquid that must be accelerated during a scanning exposure. This requires additional or more powerful motors. Further, the large amount of liquid may lead to an increased turbulence. Other systems propose the supply of liquid only to a localized area of the substrate between the imaging system and the wafer. Such a system, e.g., when using water as an immersion medium, requires a hydrophobic surface in order that the fluid meniscus is not lost during the scan.
The present disclosure is directed to various methods and systems that may avoid, or at least reduce, the effects of one or more of the problems identified above.